1. Field of the Invention
The present invention relates generally to a molding apparatus. More particularly, it relates to a mold for encapsulating semiconductor device packages having improved reliability due to reduced formation of air traps in the encapsulant. The presence of air traps is reduced by equipping the mold of the present invention with projections inside the mold cavity, extending along the sides of the lower half of the cavity in the direction of flow of the molding compound.
2. Background of the Related Art
In fabricating lead-on-chip (LOC) packages, a lower surface of the inner leads of the lead frame is attached to an upper surface of the chip by way of a double-sided polyimide adhesive tape. Each one of the bonding pads on the chip is electrically connected to a respective corresponding one of the inner leads on the chip by way of electrical connection means such as bonding wires. Thereafter, an encapsulation process using encapsulants such as epoxy molding compounds is performed to protect the chip, inner leads and electrical interconnections from exterior environmental stresses and to form a package body.
A mold is used to encapsulate the semiconductor package assemblies and transfer molds are employed for carrying out the encapsulation using a molding compound. The term `mold` used throughout the present application refers to transfer molds. In particular, the present invention will be described in relation to a mold adapted for encapsulating LOC packages, but is not limited thereto.
FIG. 1 is an exploded perspective view of a conventional transfer mold for encapsulation of semiconductor device packages; FIG. 2 is an enlarged perspective view of the part `A` in FIG. 1; and FIG. 3 is a cross-sectional view taken along the line `3--3` in FIG. 2.
With reference to FIG. 1 through FIG. 3, a mold 100 comprises a mold body 70 having an upper mold die 50 and a lower mold die 60, and a press 40.
The press 40 has a support plate 20 which is mechanically coupled to a vertically movable transfer means (not shown); a cylinder 10 integral with an upper surface of the support plate 20, the cylinder containing an oil 35 and being provided with a hydraulic inlet port 12 and a hydraulic outlet port 14; and a press cylinder rod 30, an upper part of which is within the cylinder 10 and a lower part of which extends downwardly from the cylinder 10.
The lower mold die 60 is mounted onto and fixed to a base part (not shown) located at the lowermost part of the mold 100. The lower mold die has a chase 62 for receiving molding compound tablets; a plurality of runners 64 in communication with the chase 62; a plurality of gates 66, each gate integral with a corresponding one of the runners 64; and a plurality of cavities 68 in communication with a respective corresponding one of the gates 66. The package assembly has electrical interconnections between each component and is encapsulated within the cavity 68. The runners 64 and gates 66 are passages through which the molding compounds pass so that cavities 68 are filled with the molding compounds.
The upper mold die 50 is mechanically coupled to a vertically movable transfer means (not shown), for example, at its upper surface. The upper mold die 50 has a port 52 where molding compound tablets are loaded. The upper mold die 50 has a symmetric structure to that of the lower mold die 60, including a plurality of cavities 58, but excluding the gates 66 described above.
The operation of the mold 100 will be described hereinafter with reference to FIGS. 4A and 4B.
FIG. 4A is a perspective view depicting a lead frame within a transfer mold and FIG. 4B is an enlarged view of portion `B` of FIG. 4A; FIG. 5 is a perspective view depicting the package assemblies after the molding process is completed; FIG. 6 is an enlarged perspective view of the part `C` in FIG. 5; and FIG. 7 is a cross-sectional view taken along the line `7--7` in FIG. 6.
Now, the molding process and the operation of the mold 100 will be explained with reference to FIG. 4 through FIG. 7.
Lead frame strips 200 are placed into the cavities 68 in the lower mold die 60. The lead frame strips comprise a plurality of lead frame units, each lead frame unit having a chip 110, inner leads 142 attached to the chip 110, and electrical connection means such as bonding wires 160. As shown in FIGS. 4B and 7, each lead frame has a plurality of parallel inner leads 142 which are placed onto two opposite sides of an upper surface of the chip, and the chip is attached to the inner leads by way of adhesives, for example double-sided polyimide tape 150. Further, the chip is electrically connected at bonding pads 112 to inner leads 142 by way of electrical connections such as bonding wires 160. Outer leads 144 formed integrally with the inner leads 142 remain exposed to the outside of the package body after the molding process is completed and are electrically connected to external devices such as printed circuit boards.
The upper mold die 50 is lowered by a transfer means, which is mechanically coupled to the upper mold die 50, until the bottom of the upper mold die 50 contacts the upper surface of the lower mold die 60. Thereafter, molding compound tablets are introduced into the port 52 in the upper mold die 50.
The pressing means 40 is lowered until the bottom of the press cylinder rod 30 contacts the tablets or reaches a specified distance from the tablets. The lowering of the pressing means is accomplished by a transfer means coupled mechanically thereto.
Oil 35 is charged into the cylinder 10 through the hydraulic inlet port 12, pressing on the press cylinder rod 30, thereby pressing down the pressing means 40. The lowered press cylinder rod 30 presses the molding compound tablets which are in a molten state so that the fluid molding compound 80 flows into the port 52, runners 64, gates 66, and cavities 58, 68. The molding compound tablets can be melted by the action of preheated mold dies 50, 60 having a temperature from about 170.degree. C. to about 180.degree. C., or the action of a separate heater. The encapsulated lead frame units 90 are encapsulated so that the chip 110, inner leads 142 and electrical interconnections 160 are covered with the molding compound.
After the molding process is completed, the pressing means 40 and the upper mold die 50 are elevated by the transfer means, and flow of the fluid molding compound 80 is stopped. Then, the lead frame strip 200 is removed from the mold die.
FIG. 8 depicts a schematic model of a transfer mold used to study molding compound velocity and air traps within the mold; FIG. 9 depicts the result of simulations for determining the molding compound flow within the transfer mold; and FIG. 10 depicts the distribution of air traps in the upper and lower mold die of the cavity, which is obtained from the result in FIG. 9.
With reference to FIGS. 8 through 10, the model mold having a cavity employed for the simulations has a size of 1.0 mm.times.13.00 mm. All the components within the cavity are omitted for simplicity. The molding compound runs through a common runner R.sub.1, and flows into the cavity C.sub.1 via the gate G.sub.1 and then flows into the next cavity C.sub.11 via the gate G.sub.11. The cavity C.sub.1 is located closer to the port 52 (see FIG. 4A) than the cavity C.sub.11.
The shear velocities .beta..sub.1 and .beta..sub.11 in the lower mold die are greater than the shear velocities .alpha..sub.1 and .alpha..sub.11 in the upper mold die, since the flow channel in the upper mold die is narrower than that in the lower mold die, and the molding compound flowing therein undergoes a greater flow resistance. Further, since the molding compound is introduced into the lower mold of the cavity after being introduced into the upper mold of the cavity and the shear velocities .alpha..sub.1 and .alpha..sub.11, and .beta..sub.1 and .beta..sub.11 are measured at the same time, the shear velocities .beta..sub.1 and .beta..sub.11 in the lower mold cavity are inevitably higher than the shear velocities .alpha..sub.1 and .alpha..sub.11 in the upper mold cavity.
As a result, air traps T are formed where the molding compound does not flow uniformly, and where the molding compound does not fully fill the cavity, thereby causing air bubbles.
The partial filling of the molding compound and the creation of air bubbles may reduce package reliability. Specifically, water vapor is introduced at the partially filled portion and/or air bubble portion during the package reliability tests which are performed at elevated temperature and pressure. The water vapor causes separation between the inner leads and the chip and electrical shorts at the bonding wires.
The air traps T produced within the encapsulated package are localized at the positions having the greatest velocity deviation along the shear velocities shown in FIG. 9. Moreover, more air traps T are observed at the positions farthest from the gates G.sub.1 and G.sub.11 of the cavities C.sub.1 and C.sub.11. Note that the small and large circles in FIG. 10 simulate the small and large openings of a lead frame during the simulation of the flow of molding compound within the transfer mold.